Telephone system



Oct. 7,1941, A. A. LUNDs'rRoM TELEPHONE y SYSTEM Filed oct. 11, 1940 IWI. a T N 11% h. I; Ik M .Omll

Hlll

/N'VE/v To@ Y ,4. A. UA/Ds TRO/v A T Tom/5y v mme; i, 11 l l l ma EQ2 mh, A-H-)Q 1 J Patented Oct. 7, 1941 UNITEDv A STATES PATE Nr oFFlcEi Y i 12,2518@A y TELEPHONE ASYSTEM AlexisiA.- Lundstrom; East Orange, N. J., assignor to Bell` Telephone Laboratories,y Incorporated, .-New vYork, N. Y., a corporation of New York Application October 11, 1940, Serial No. 360,717'

7 Claims. l(111."-179-18) This invention relates to telephone. systems and particularly to the receiving equipment of,l a transmission line over which alternatingH currents, preferably within the voice frequency range,

Inaccordance with a feature of this invention, the channel` detectors are protected against false operation by the use of two volume limiting -devices interposed betweenthe line and the receivare transmitted for effecting lthe Yoperation of 4V'15, ing channels thevr first of whichA operates when said equipment which controls, in turn, the se. ever the level ot the signal energy-isbn one siglective positioning of switching apparatus. yMore nal-,5y a predeterminedl number of energy incre. particularly, the invention isin thenature 0f mentsabove the predetermined signal level to an improvement in the receiving. equipment. of which the .channelsA are adapted to respond, to the telephone transmission line disclosed in the 1Q lock-inf aidenite lossso` as to reduce the signal copending application of A. A. Lundstromerial to within one or two increments above thepre- No. 355,712, filed September 7,1940. Insaid' ap. determined signallevel, after which it,` becomes plication is disclosed a telephone systeinof the unresponsive to further signals, and thesecond type in which automatic-switches are controlled volume limiting device operates on Ievery 4signal by means responsive to alternating currents of to introduce'a further adjustment Wherebvtle different frequencies generated at a subscribers level o f the signal is further reduced to the level station and transmitted over a line which is proof one increment above that to which the chanvided with a plurality of channell detectors each nels are adapted ilo-respond. i selectively tuned to a different one of the fre-' In accordance With the invention one embodi#A quencies and responsive thereto to operatej a re- 2,9 ment ofwhich is disclosed herein by way of illus,- lay which, in combination with a relay operated tration, a transmission line is provided Over over another channel detector, serves to function Which signal energy that fiuctuates over a wide a settable register, or sender, in which Vis lockedranger may be transmitted. Y ConnectableA to the in a record of the digit indicated by the two op` line is ya plurality of channels, each Vof' which is erated relays; that is,` by ,the twol frequenciesy adapted to respond to a particular frequencyin transmitted from the subscribers station;V the Signal enelgv, while disposed between. fh@ In systems of this kind Where thefsignal genchannels and the line are two volume limiting erating instrument is, usually, a, plurality' of keys' devices the rst of which is adapted to respond or the like which are operated to produce the dif-j to any level of signal energy substantially above ferent combination offrequenciesV to which the' th levelto which the channels are adjusted to. channel detectors are 'respectively tuned .to re;I operate'for the purpose of reducing the' Same spond, it is difcult to keep the transmissionline to5ava1ue slightly higher than thatV to which'the completely freev of all frequencies'exceptthe. SigQ- IldiVdllal Channels are adjusted O'ODeIal'fe. The nal frequencies generated. From' an engineering secondvolume limiter alsorespondsv to the signal standpoint it is very desirable to have the 'signal ,35 energy but functions only to reduce what is left frequencies within the voice frequency range and', 0f the energy after the' Vfirst, volume limiter' has therefore, noises of all kinds will include said sigacted upon it to. further reduce the energy to the nal frequencies. 'It has been found, for example,l predeterminedl level'to whichV the channels are that one of the sources ofsuchnoise'is'thecure adapted to respond. In bothv cases thevolume rent carrying contacts of Vthe -relays' vvhichjestalcfLQ limiters act to obtain this' result byintrodu'cing lish the transmission line itself between the teleappropriate losses in the line which subtract'by phone station and the receiving channels.` Itjis that much' from the total available energy; the ihOllght that this HkOSe is DIOddCed bythe 51112111" total losses in all cases being such as to leave aV VarialfOIlS iIl the quantity 0f Currents Passing residuum for the channels which `is at the-prethrollgh the Contacts, and Since the frequency $5' determined energy level for which theyv arcadspectrum Aof, the noise usually contains cormpo-A ju'sted to spond I ments of the dlerent Slgnal fregFengF-Sf the YThe invention will be more readily-ascertained. channel detectors, or some of them, are liable to from-the followin d t .l d d t. l f respond to'such vfrequencies and therebyprI t. g" .e 271e escllp .lon ,of sam" duce a false registration even though thesgnal 45:0 mven ion, appended claims and attached drawgenerating instrument may or may not havebeenI operated to produce frequencies other than those produced by the noise and therefore-'cause the operation' of' other channelsr beyond vthoses'in-y tended;

f ings in which is shown schematically certain ele.

ments of the-telephone system'morefcompletely disclosed in 'the copending application of A; A. Lundstrem,` Serial No. 306,466, fledNovember 28,"

V5.5.- 1939, and in which certain othe'relements; to

which my invention more directly relates, are shown in detail.

Referring, now, to the drawing, S represents a subscribers station in an automatic telephone system, said station being equipped with a device I suitably arranged to produce one or more currents of different frequencies in the voice frequency range in response to the depression of keys or the like that represent digit or other characters making up the wanted designation; 2 represents a line extending from the station instrument to the central oflice whence, throughm one of a plurality of selectable sender connectors i3, the line is extended to a free sender which comprises volume limiter A, volume limiter B, the enabler Ii) and the channel detectors f I, f2, f3, f4 and f5 individually so designated in accordance with the frequency to which each of the channels is respectively assumed adapted to respond, which channels include suitable filters (not shown), individually responding relays and the settable register relays which follow the operation of the channel relays when operated to signalize an incoming impulse. The amplifiers 8 and 9, the vacuum and gas-filled tubes in the various portions of the apparatus as Well as the coupling units and rectifiers are commercially available circuit elements while the channels I 5 are completely disclosed in the copending application of A. A. Lundstrom Serial No. 306,466, to which reference is made for a more complete understanding thereof although it is to be understood that the invention is not limited to channels of this description.

In the operation of the invention, a call is initiated at station S by the removal of the station instrument from theswitchhook, the transmitter thereof being short-circuited or removed from the line in any suitable m-anner to prevent unwanted speech currents from entering the line, whereupon the lineis then extended by any suitable means to the central omce at which an available sender is then connected to said line through the medium of a sender connector I3 after which relay 45 is operated in any suitable manner and key tone (not shown) is transmitted back to the calling station S as a signal that the sending device I may be operated to transmit the number of the wanted station, al1 in the manner described in the last-mentioned copending application of A. A. Lundstrom. Relay 45, in operating, connects positive battery to the top winding of polarized relay 340i volume limiter B which is serially connected to the top windings of polarized relays 3l, 32 and 33, the last-mentioned relay having the other terminal of its top winding connected to the plate and screen grid of vacuum tube 36. Now the lament of this tube is permanently connected to a local current source (not shown) and the cathode thereof is, therefore, permanently activated so that a circuit is completed through the top windings of all the above-mentioned relays, the plate and cathode of tube 36 to ground. Relays 3i, 32, 33 and 34 operate since current flowing in this circuit overcomes the current in the series circuit through their lower or biasing windings, relays 3l, 32 and 33 disconnecting resistancev pad device I is operated, two out of the five defined frequencies fI-f5 are produced in the combination required to designate the digit represented by the key, and the currents thus produced are transmitted over a circuit which may be traced from one side of the sending device I, lower conductor of the looped pair 2, lower back contacts of relay I2, next to the inner contacts of sender connector cut-in relay I3, primary winding of transformer 3, inner contacts of relay I3, top outer back contacts of relay I2, upper conductor of looped pair 2 tothe other side of the station sending device I. The current composed of the two frequencies generated by the device I as well as current of the frequencies arising from the contact noise as mentioned above, are induced in the secondary winding of transformer 3 and are fed into the amplifier 8 whence, after suitable amplification, these currents are fed, via the primaryof transformer 4, and via the primary and secondary of transformer 5, into amplifier 9. Y'

Considering that portion of the signal energy which passes through transformer 4, said energy is applied undiminished to transformer 43, whence it is transmitted to transformer 42 for application to the amplifier tube 31, the output of which ,is set in accordance withthe voltage drop developed across resistance 38 applied to the grid of said tube asa bias therefor.

Upon the arrival of the first signal impulse several conditions are to `be distinguished. If it is supposed that the voltage of the incoming signal, as determined by the .output of tube 3'l is, say, between Eo-Ei which may be assumed to be two voltage limits, thel first (Eo) defining the just'operate voltage of the channel detectors )1 -f5, while (E1) defines the maximum voltage Athat the detector-s will tolerate without falsely operatingadjoining detector channels due to unwanted transients from the signal and limitations of the selective tuning kof the channels, then neither volume limiter A nor B will operate, by which is meant that volume limiter A will not operatebeyond the operation of relays 3I-34 to remove the pad elements 41, 48 and 49 across the. line when *thev sender is connected thereto as'already described. In this case the signal voltage Eri- E1 is applied to the input winding of transformer 40 and Vrepeated in the loop comprising the secondary of said transformer, the primary of transformer 54, and the primary of transformer `I whence it is applied through the secondary winding of rtransformer l to thereelements 4l, 48 and 49 respectively from across the line by causing their armatures to engage their respective left contacts as shown before any signalsare vreceived while relay 34 connects the grid of tube 33 to the anode of gas-filled tube 53 in volume limiter A. Y

When any one of the ten keys of the sending sistance pad I4 and through it tosuitable ltering devices (not shown) and to the grids of the detector tubes in each of thechannels fI-f. Since these tubes are rnormally inactive those channels that would be responsive to the two frequency components in the signal impulse transmitted through the resistance pad I4 do not operate until ground is connectedV to the cathodes via the contacts of polarized relay 20 and conductor I5 as more particularly explained hereinafter. Further, `a portion of the signal voltage is also repeated into the secondary winding of transformer 54 the two terminals of which are connected to the-control electrodes of gaslled tube 53, but this voltage is not suicient to break down the tube and no further operations result therefrom. The full voltage Eo--E'i is further applied tothe secondary winding of transformer 4I the two terminals of which are connected to the control electrodes of gas-filled tube 39, but here again, as with tube 53, the

' the" tube 'and' no1 further 'operations esult.

. The `Anext condition to "bef `distiri'g`u`ished l"from 'the one"abel/efdiscu-ssedis'thalfarising` from the Y 'assumption that the-signal Ampjui'sevoltageavail- `'f 'alole'i 'on 1 the l 'primaryfsjide "of transformer Q40 ""is 'between'ElEz' by "which'is meant that the intensity of thej'signal is higher 'than they'all'ie'raslsunfie'd 'under the r'st condition. "Snceit 'slffur-i they assumed thattliesigrialimpulsen tn first of f the' senesjwliieh" is to designate theiwanted humilier, and," therefore, the voltage* ce* ltine-1 impulse for this bartlculafeall' will not rise-'beyond Ea since 'the terminal-g signal v oltage isf'affunc- "tin f!4 the resistance Yof' the calling loop; Volume 'lectrodes' `of gas-filled .tubes 39" and -thefi'lalue 'of ofi this tube 'and cause 'it' to be'coiifiev conducting whreupon theande' 'thereof app'lies'a 'negative Bias potential tothe grid "of 'l'acl'lun tube 35.

l'and 'resistancelv 58,I thecondlictibn "cffthe 'tube causesl a negative yoltage'in respe'ct'tq'ground to be established across condenser y'57 'whi'ch acts asa 'negative bias to the grid'and thus causes a elementwi and resistance feleinenfl. 'Under norrn'al 'conditions' 'the resistance'fbetween ele-v "ment 59 and ground thr'ough'the Acath'd'e'fari'ode circuit of the tube is Vsuch thatthje'is' no potential differencebetween the"tehnilfral''joining resistance eleinentsf '60' and 6 l and, thatj'oning reinstance element 59j and theiaoie. v'c'crjiseqlient-k Y 1 ly,under' 'the 'eliminen that the intensity' of ythe signal impulse is between Mlet-'Ei no "potential dile're'llle isl applied betweenl the till/'0' tIIi-flls of the Varistor bridge 46`and, ytherefore;this bridge draws no lcurrent through it to establishy a resistance loss across conductors 62'and 63.: This means that'if the signal is withiny the normal Voltage Eo-Ei thatl Voltage is 'applied undiminished to theterminals ofthe primarywinding of transformer BZWherefrom it is'tra'nsterredh to transforme-r 'I via transformer 4lill for applca` tion to the pad i4. Now if the signal impulse is higher in Voltage;` that is,"betw'e`en"/E1'-` E2, the current through the anode-'cathode "circuit l'of 'tube' '35 is V'reduced as above set forth, a; potential di'ierence'is established bfetween resistance' l elements 6 6 and 6I on theorie side' and'resistance element' and the ":cathodeanode"circuit' on'the other which is applied tothe terminals 'ot the Varistor bridgev 46, thereby 'reducingitsfeiective` A resistance and establishing a shunta'crossj con-f ductcrs B2 and r63 the effect' ofwhich'is toreduce Athe potential of rthe''signal'impulse'to the 'Value AE1 and lto hold it to 'this value 'for the duration f the Sign-ail" :f .lv-, en the 'signal terminates, tube 39"1sex y `"llftisuiosus'taiirii'ng'ivdltage, ithe rr nt th ugh*yacuumitubeisjlreri'ts-n "mali'ralue and thefnotentialdifvfe nce between"t h "twot'erminals of theivaristor 2 45 A is "eliminated *thereby 'remyingfthe Ycd Vfaa y ,"ossljthe fcqnductorsz and-L63.

ignali impu1sewhich-comes ltage* value Etf-"1.12 will cause Y? peratnbfvolumelriter'B d above-toi reduce the' value of `the''sig- 'I u mp lseydltage'vto be applied-to the channel *detectors Jtcl-*theVallie'fEl. ,Y Y p *y unposef'nowf thatfthe 'signal 'Voltage` available l the transformer 40' is `between"E2-Es"fwhich rluef isfassumedftolbe 'higher' than E17-E2 and high endugh' to lcall iOrjthef connection lofa perjnianentlossffo "the: line. A"Uriderthes'e circumpstances, yuitje-39will-:again*operate but-in lthis 5 'case,jthe'"vdltageLai/ailable' attransformercf is rsuil'icier'lttocauseabreakdown of gas-filled tube 53which, since relay 34 is operated, willjn turn lfa'pplyra negativebiasing potential across the ter- F"rnjinzvilstof cond einser` 52 for -the'A grid 'of Itulle 36 v'viatheianodelof'tullei53and resistance 64. Since '5 current i' isf4 normallyrflowingf throughv the anode 'circuitofvacuumtubel36j asfdescrib'ed above, the j establish'rnentof 4 the vI'legative loiasto the grid `"thereoffrhas4 thev effect lofjreducing the current flowing tl'lloughlv "thiscircuit-which also includes 3o the topewindings of relaysY 3 misa inclusive. The

"reduction in currentis 'suiiicient lto cause Athe curfljentj `throlzlgh the" lower'"biasing"V windings of reflaysj 3 I` 'and-1 34' to overcome the reduced current liowing lseri-allyr throughs vtheir "top windings, `v'vher'eupon lthe"armature 'of' relay' 3i vmoves into engagement with `its lright-contact and introduces *pad* 1 1finto'tl'lelline'-toy reduce-the intensity "of Aftheyincorningsisnai-td a' Value lower Athan' E2" but greater' 'thanf E1. vVoll'linelimiter B, however, tis "concurrentlyfoperating in the manneralready Y'de'scribed'jin reducelgesignal voltage" 'frm `any Value between E2A aiidEftdEl 'so that 'the'signal L yoltagiayailable'-lat-the channel detectors :is lno higher-thans. *Relay 34 jijeleasesandlopens Kthe charging'path In jtubetq condenser `.521 which, in turn; has leffect"oflniaihtainir'lg"the'charge'on said'c'on- "jd-enserffor'the duratipn'jof the entire :signal period thatis; for )the remaining signal "impulses stillj' to "be "frthcoming. This relay" 34 `isy locked l in a releas'd'fconditiomso tdspeak,finasmuch as 'the'anodefcurrent flowing' through tuloe` 36 and the-tgp win'dlngsifof relays M434 is reduced by Athe'permanent'biaspotential applied to thev grid sa-thereof?byjthepermanent Icharge condition" on 1 condens'erj'152. /Tuben53 asi'wellas tulle`l 39;'exf Atinguish when the Ysignal impulse is terminated.

this f way, 'j'pad 4T isloeked in acrosst the jjjtrans mission'linefforfr the duration oi?v the" sig- 60 j nailing e perigd and all subsequent sign-als which -t'come' in will 'have' voltages' Eij-E'zwhich'V volume Llirniter` kBf will 'reduce to value El forapplication tot tnechamieledtectors flffz.

4-If-the initial signalfis,'say,`between E3-E4 sfwliich Tis'iassumed' to vvheliigher than Eze-E3, then 1 since relayj34f iisv operated? and tube" 53 will Vbe V Y:rende'red" j''or'lduting'.by theportionof the voltageA available" 'at transformer 54, the anode 'cur- "f'rent applied tof condenser 15,2 is higher than'when `r`*"(ithe` signal impulse" had' a*po tentialjv'aluej Ez-eEa,

"'the" negative charge'cn `cclnden'ser '52' is correspondingly higher, the negative bias forthe grid :of tube 3h: is;V therefore, 'also l'igl'ier andthe ani* "de l'cli'rrent'v 'wng through' tube 36? and 'the "l 5 'upper 'windings ior s |434J is 1liow/minced to the value where not only relays 34 and 3| will release but also relay 32, whereupon the varmature of relay 3| will engage its right contact to insert pad 41 in the transmission line and the armature of relay 32 will engage its right contact to insert pad 48 in said line. Pad 48 reduces the voltage impulse to a point between E2 and E3 and pad 41 to some value between E1 and E2 while volume limiter B further reduces it to Ei.

If the signal voltage is, say between Eli-E5 which is assumed to be higher than E13-E4 the same operation as above described will take' place except that, now, the anode current of tube 35 is further reduced to allow therelease of relay 33 whose armature now engages its right Contact to insert pad 49 in the line the purposes of which is to reduce the impulse voltage between E3 and E4. Thus in all cases volume limiter A always reduces the signal voltage to some value between E1 and E2 while volume limiter B further reduces it to E1.

When the sender has received its full complement of signal impulses, relay 45 is released in any suitable manner, thereby disconnecting battery from the upper windings of relays 3I-34 causing the same, under the influence of the current flowing permanently through their respective lower windings, to swing their respective armatures into engagement with their respective right contacts and incidentally place in the transmission line the pads 41, 48 and 49. Since the circuit is not in use when relay 45 is released this disposition of the pads is immaterial to the operation of the circuit. Ground is further applied through the back contact of relay 45 to the upper terminal of condenser 52 causing the same to be discharged in readiness for the next call.

Considering, now, that portion of the signal energy which is transmitted at each impulse to the enabler circuit I0 via transformer 6, it will be observed that said enabler comprises a rectifier bridge I6 and four polarized relays I1, I8, I9 and 20, each having a permanently closed polarizing circuit through its lower winding which causes the armature thereof to assume the position indicated in the drawing. Further, between each two relays, beginning with relay I1 and ending with relay 29, there is an intermediate condenser-resistance network which joins a contact of the relay on the leftwith the upper winding of the relay to the right. Considering relays I1 and I8, for instance, the network made up of resistances 22 and 23 and condenser 2I joins the right contact of relay I1 with the upper winding of relay I8. Since the armature of relay I1 is connected to positive battery 39, condenser 2I is normally charged through resistance 22 to the potential of this battery. Condenser 24 of the network between relays I8 and I9, on the other hand, is normally atground potential since it has ground applied to it through the left contact of relay I8 4and resistance 25. Contrariwise, condenser 29 of the network between relays I9 and 29 is charged to the potential of the battery 39 through the upper winding of relay 20 and resistance 28 since the short-circuiting ground path for condenser 29 is normally open at the right contact of relay I8, which fact leaves condenser 29 exposed to the charging circuit that traces from the positive pole of battery 38, upper winding ci relay 20, resistance 28, condenser 29 to ground.

Assuming, therefore, that relays I1, I8, I9 and 29 have their respective armatures in the positions shown, and that condensers 2|,L24and 29 are charged, discharged and charged, respectively, as indicated above, ground is normally disconnected from conductor I5 at the right contact of relay 29, which conductor is, in turn, connected to the cathodes of the detector tubes of the sevveral channel detectors fI-f5, and causes them to be normally unresponsive to signal energy incoming over the resistance pad I4.

Now when a portion of the incoming energy is applied to the rectifier bridge I6, said energy is rectified and the current thereof is applied to the upper winding of relay I1. The combined effect of the current now flowing through the upper winding of this relay and that always flowing through its lower winding is such as to cause its armature to swing from the right contact into engagement with its left contact. Since condenser 2I is normally charged to the potential of battery 39 through the armature and right contact of relay I1, the opening of these contacts immediately causes the condenser to discharge to ground through resistance 23 and the upper winding of relay I8 to prolong the flow of current normally flowing through this relay winding. The direction of the discharge current flowing as well as the normal current through the upper Winding of relay I8 opposes that of the current normally flowing through its lower winding, and since the discharge current is initially of greater intensity through the proper control of the ohmic value of resistance 23, the release of relay I8 is delayed and the armature thereof is caused t0 break away from its left contact at a time subsequent tothe operation of relay I1 and to engage its right contact.

During the time that the armature of relay I8 is engaged with its right contact, and remembering that condenser 24 is kept at ground potential while the armature of this relay is making with its left contact, then, as soon as the armature breaks with its left contact, condenser 24 immediately begins to charge over a circuit traced through resistance 26, the upper winding of relay I9 to battery 39 to continue to maintain the flow of normal current over this path for -a short time subsequent to the operation of relay I8, whereupon the effect of the current through its lower winding continues to be neutralized for a short time by this changing current rather than by the normal current and the armature of relay I9 is thus caused to breakwith its left contact at -a'time subsequent to the operation of relay I8.

VAt the instant that the armature of relay I8 engages its right contact and before the armature of relayV I9 leaves its left contact, a discharge pathis completed for condenser 29 which traced from ground, condenser 29, resistance 21, left contacts of relay I9, right contact of relay I8 to ground. The energy accumulated on condenser 29 as a result of the charging circuit through the upper winding of relay 20 is quickly drained off through the low resistance 21 and a current flow is quickly established in the upper winding of relay 29 to cause it quickly to operate and connect ground to conductor I5 to sensitize the detectors, so that when the armature of relay I9 breaks its left contact, condenser 29 is in an uncharged condition and immediately starts recharging through the upper winding of relay 29 to prolong the current ow for a subsequent period. This charging current overcomes the effect of that normally flowing through lower winding of relay 20 and causes the armature thereof to engage its right contact, thus applying .ground to conductor I5 which, as said before. .is connected@ thecathode :of @schoof .the detectorl tubesinthe channel detectors fI- -f5. The channeldetectorsare now made Yresponsive andihetwgwhich areadaptelito respond, t the two frequencies in the inputr current thatows into their respective input side through resistance-pad Id operatein turn causing the operation 'of their respective anode relays which, in their turn, cause the operation vof. relays in the register which` designate the digit or character marked by the two frequencies. v

It is 'to be noted that relay I8 will makeiits right contact at a time subsequentto vthe energizing rof rrelay I'I by the current through vrectifier bridge I6. 'This time is, of course, determined by the discharge timejof the .condenser 2I-through resistance 23 which causes the normal current through relay I3 ltoiendurefor a time (let us say, for example, 20 milli`sec-'orlds), after the operation of relay I?. At this time'relay I8 releases and closes its right contact whereby condenser 29 is quickly discharged through the low resistance 2l and-establishes av current through the upper winding of relay 20 tocause its immediate operation. .'Ihis,` as explained, places ground on conductor I to sensitize the detectors'at a time 20 milliseconds after thesignal was first applied. This delay is for the purpose of allowing the unwanted spectrum energy in adjoining channels to dissipate itself in order to avoid'unwanted'channel roperation from this source. It is the purpose ofthe condenser 2e to maintain, by its discharge current through-the upper winding of relay i9, thev normal current flowing to hold relay I9 operated on its left contact for a time suilicient to permit the'co'mplete discharge of condenser 2S through resistance '27, left contacts of relay i9 and right contacts of relay I8 to ground. Relay I9 then operates to break its left contact andl thereafter relay 2Q is maintained on the charging current from battery 3U, top winding of relay 2 0, resistance 28, tocondenser 29. When this 4discharge current has dissipated itself, relay` will operat' and break its right contact to removeground from conductor I5 regardless of how long thereafter that energy is delivered to relay I'I by the signal. The length of time during which the armature of relay 2i) engages its right contact to cause the channel detectors to remain activated depends, of course, upon the constants of condensers 24 and 29 and resistances 26 and 28, whichvmay be chosen to provide any interval desired, mostk of the time being determined bythe charging time of condenser 29 through resistance 28. 'Howeven when the condenser 29 is completely charged, the current through the lower winding of relay 20 again becomes effective, the armature of the relay breaks away from its right contact and ground is 'removed from conductor I5 and, therefore, from the cathodes of the detector element of the several detector channels which, in consequence, will no Vlonger respond to any signalenergy Vforthcoming over resistance pad I4 and thereby causey their separate anode relays to release. l i A,

The circuit conditions prevailing in the enabler duringvthe presence Vof signal energy in bridge I6 `and after the armature of relay 20 has again made its left contact, is that relay I'I will have its armature making with its left contact, relay I8 will have its armature making with its right contact, while relay I9 will have the armature on its right contact which will in no way aiect the charge on condenser 29 or the conditioniofthe armature of relay 2i! which is out of engagement with its right contact.-V Hence it makes no difference how long signal energy prevails upon the vline subsequent to the armature of relay 2li breaking away from-itsback contact, said energy will vbe ineffective to operate the channel detectors inasmuch as groundis disconnected from the cathodes of their several detector elements. This feature is of chief value in avoiding the false operation of adjoining channel detectors due to spectrum transient set up at the channel inputs when `the signal is suddenly removedf @y j When the signal impulse is oven-relay III will releasesv reestablishing quickly therebyV the charging circuit of condenser 2| and the current through the top winding yof relay I8 to cause it to make its left contact. Relay Iii-operates and causes 'the quickA establishment of` 'current through the top winding of relay Ito cause it to quickly make its left contact. In this operation thev condenser 29 has not 'been discharged since ground via the contacts of relays E8 and I@ has not been made.V The enabler circuit is now ready for another cycle of identical operations' upon the reception of the neXt impulse.

While we have described our'invention and the means for utilizing the saine in connection'with its specic application to a particular kind of a transmission line, it is to be understood that various other applications land embodiments thereof may be made by those skilled in the art without departing from the spirit of the invention as dened within the scope of the appended claims.

What is claimed is: i

1. The combination with a transmission line adapted for vtransmission thereover of signal energy fluctuating over a wide range, of a plurality i of channel detectorsfeach adapted to respond to a signal of a deflnitefrequency and predetermined energy level, and two volume limiting means interposed between said linev andk said channels of which one is responsive to an incoming signal having an energy levelvhigher than said predetermined level to reduce said signal to some value within a defined energy vincrement above thepredetermined leveland of which the other is responsive t'o the same signal to further lreduce the level thereof to said pred-etermined level.' v,

42. The combination with a transmission line adapted for transmission thereover of signal ener'gy uctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a denite frequency and predetermined energy level, andtwo volume limiters both interposed between said line and said-channels of which one is responsive to an incoming signal having an energy level higher than said predetermined level to reduce said signal to-some value within a defined energy increment-above the predetermined leveland the other of said Volume limiters is responsive to the same signal to further reduce the level therof to said predetermined level, andmeans lin said rst volume limiter for rendering said limiter unresponsive to the-other succeeding signals. Y

3. The combination with a transmission line adapted for transmission thereover of signal energy fluctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a definite frequency and predetermined level, and two volume limiting devices both simultaneously responsive to an incoming signal having an energy level higher than said predetermined level for reducing said signal to said predetermined level, one of said volume limiting devices comprising a thermionic device responsive to said signal, a plurality of relays, a plurality of resistance elements controlled by said relays for connection into said line, each of said resistance elements having a value capable of reducing the signal level by some determined energy increment, and means responsive to said thermionic device for operating one or more of said plurality of relays for connecting a corresponding number of said resistance elements into said line to reduce the level of said signal by as many determined energy increments as there are elements connected.

4. The combination with a transmission line adapted for transmission thereover of signal energy fluctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a deiinite frequency and predetermined level, and two volume limiting devices both simultaneously responsive to an incoming signal having an energy level higher than said predetermined level, one of said volume limiting devices comprising a thermionic device responsive to said signal, a plurality of relays, a plurality of resistance elements controlled by said relays for connection into said line, each of said resistance elements having a value capable of reducing the signal level by some determined energy increment, and means responsive to said thermionic device for operating one or more of said plurality of relays for connecting a corresponding number "i seid resistance elements into said line to reduce the level of said signal by as many determined energy increments as there are elements connected, the number of said relays operated f'feperding upon the value of the signal voltage applied to said thermionic device.

The combination with a transmission line adapted for transmission thereover of signal energy fluctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a definite frequency and predetermined energy level, and two volume limiting devices both simultaneously responsive to an incoming signal having an energy level higher than said predetermined level, one of said volume limiting devices comprising a thermionic device responsive to said signal, a plurality of resistance elements each capable when connected into said line of reducing the signal level by some determined increment, a plurality of relays for controlling the connection of said resistance elements into said line, each of said relays having a rst winding through which flows a normal current adapted to position the relay to normally connect its associated resistance element into the line, an electronic device having its cathodeanode circuit connected serially through a second winding of each of said relays, means responsive to said line being taken into use for the transmission of signals thereover for establishing a cathode-anode circuit through said electronic device including said other Winding of each of said relays whereby the current flowing therein opposes the current in the iirst winding of each of said relays for operating said relays to disconnect their associated resistance elements from said line, and means responsive to said thermionic device for reducing the current in the cathode-anode circuit thereof whereby one or more of said relays are operated to connect their associated resistances into the line, the reduction in the current of said cathode-anode circuit and the number of relays so operated depending upon the value of the signal voltage applied to said thermionic device.

6. The combination with a transmission line adapted for transmission thereover of signal energy iluctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a definite frequency and predetermined energy level, and a first volume limiter responsive to an incoming signal having an energy level higher than said predetermined level, for reducing said signal to some value within one energy increment above the predetermined level, and a second volume limiter responsive to the same signal to further reduce the level thereof to said predetermined level, said second volume limiter comprising a thermionic device responsive to said signal, a Variable resistance element connected across said line, and means responsive to the operation of said thermionic device for applying a potential diierence across said variable resistance element whereby a shunt is established therethrough to reduce the signal to said predetermined level.

'7. The combination with a transmission line adapted for transmission thereover of signal energy fluctuating over a wide range, of a plurality of channel detectors each adapted to respond to a signal of a definite frequency and predetermined energy level, and a rst volume limiter coupled to said line and responsive to an incoming signal having an energy level higher than said predetermined level for reducing said signal to some value within one energy increment above the predetermined level, and a second volume limiter coupled to said line and responsive to the same signal to further reduce the level thereof to said predetermined level, said second volume limiter comprising a thermionic device responsive to said signal, an electronic device, three resistance elements forming a resistance bridge with the resistance of the cathode-anode of said electronic device, a variable resistance bridge two vertices of which two are connected to said line and the other vertices of which are connected to terminals of said resistance bridge, means for normally producing a current flow through .said bridge whereby no difference of potential is applied to said variable resistance element connected thereto, and a network for the grid of said electronic device responsive to the operation of said thermionic device whereby said grid is negatively biased to reduce the current flowing through said resistance bridge including the cathode-anode branch of said bridge for producing a difference of potential to be applied to the terminals of said variable resistance element whereby a shunt is established across the transmission line therethrough to reduce the signal to said predetermined level.

ALEXIS A. LUNDSTROM. 

